I also had customers that were not able to install my app on an older device.
I learned then, that this rule to install an older app on a device that doesn't support the latest version of your app is limited to Apple-ID's that already once installed that app on a device. Apple probably only wants to enable users, that already had an app (and maybe paid for it), to install it later again on the old device.
If your user foolishly deleted the app from the device, then I think, he should be able to install it again when using the same Apple-ID as for the first installation.
New users that want to install the app on an older device, can use the following workarounds:
Variant 1:
If the user has another device (iPhone, iPad or Mac) available, which is new enough to install the current app version, he can install the app there first with his Apple ID.
After successful installation on this other device, he can also delete the app there right away if he doesn't want it there. It is already enough, if Apple knows that he had already installed the app once on a device with hiss Apple ID.
Now he can also install the app on an older device that no longer supports the current version of the app. He will then be asked in the AppStore app when downloading if he wants to install a version compatible with the device instead of the current version.
Variant 2:
If the user doesn't have another device (iPhone, iPad or Mac) that is new enough to temporarily install the current app version, you can provide a promotional code to install the app for free. (If it is a free app with possible in-app purchases, it doesn't cost you anything.)
Then he can enter that code on a Windows PC in the iTunes software (where he is logged in with his Apple ID). On a Mac, he can redeem the code using the Music app. (On older Mac's that still have iTunes, also in iTunes).
See also here for more options: https://support.apple.com/en-us/HT201209.
Important: Directly on the device, which is too old to install the current version of the app, the code cannot be redeemed either!
Once the code is redeemed to his Apple ID, he can install the app on an older device that no longer supports the current version of the app. He will then be asked in the AppStore app when downloading if he wants to install a version compatible with the device instead of the current version.
I hope that helps. I copied and slightly modified it from the FAQ pages of my own app.
